In such a gas spring the piston located at the free end of the piston rod is formed as a damping piston which tightly seals when the piston rod is extended from the housing. The gas flows via a duct formed in the piston rod and bridging the piston from one side of the piston to the other, and this duct is connected by means of a throttle bore with the associated inner chamber of the gas spring. The duct is formed in the piston rod of such a length, and the throttle bore is disposed at such a distance from the piston, that prior to reaching the end position of the piston rod and piston the throttle bore enters a bore disposed on the end of the gas spring where the piston rod exits, having an only slightly greater diameter than the piston rod, so that the gas can only flow through the throttle bore and the duct under additional spring throttling. Especially in the case where the gas spring is filled exclusively with gas, or where possibly oil is no longer present where needed in the end of the housing where the piston rod exits when the gas spring is in the largely extended position, the throttle bore has to be very small. Such a small throttle bore, however, is relatively easily plugged up if small particles are for example rubbed off the seals. Bores as small for instance as 0.4 mm diameter, which for technical reasons are about the smallest practical diameter, do not provide satisfactory damping.
Such gas springs are particularly used as a lifting aid for hatchbacks or hoods of automotive vehicles, and often the outer free end of the piston rod points downwardly in the closed position of the hatch or hood, whereas in the opened position it points upwardly. If oil is used in such gas springs for stop damping during the extension of the piston rod, then this oil is, at least after a first extension of the piston rod, on the wrong side of the piston fastened to the piston rod, and therefore it is not in a position to satisfactory effect the end position damping. To remove these problems it is known from German Pat. No. 24 14 457 to attach a cup-like container to the piston rod in the area of its piston attached on its inner end, which is open in the direction towards the exit of the piston rod. A liquid-level displacement float is associated with this container and is attached at the end of the housing where the piston rod exits. The outer diameter of the container is smaller by an insignificant amount than the inner diameter of the housing. If the end of the housing where the piston rod exits attains an upwardly directed position when the hatchback is swivelled upwardly, the damping fluid present generally flows into the container and is carried along by it, until the liquid-level displacement float dips into the container and pushes the damping fluid out. This then is forced out through the gap between the container and the inner wall of the housing, to provide a corresponding damping. This structural solution is very complicated. In most actually occurring cases, the gas springs are installed as lifting aids for hatchbacks or hoods, such that in the closed position of the hatches the outer free ends of the piston rods point upwards, and in the opened position of the hatch they point downward. In this case the oil contained in the gas spring cannot be practically used for end damping when the piston rod is extended, unless special steps are taken.